Because the term Agnostic confuses Christians. A Christian would think that I hadn't yet made up my mind on the existence of Jehovah or the Messiahship of Christ. I have and I absolutely believe in neither of them. Agnostic also denotes weakness whereas Atheism denotes certainty.
I am aware of the common nomenclature of Strong Atheist and Weak Atheist and I reject them both. Not only is this description unnecessarily demeaning to the so-called "weak" atheist but it ignores some basics of the belief systems. I have created my own series of descriptors.
Empirical Atheist - Extremist
The Empirical Atheist Extremist is one step beyond empirical atheist and denies not only gods and the supernatural but any possiblity of alien civilizations, life in space, powerful beings, other dimensions or anything we have not thought of already. The Empiricial extremist knows that we in the 21st century have all the answers to everything and you're a total idiot if you question any current scientific theory.
Empirical Atheist
Represented by most of the scientific community, an empirical atheist denies even the most remote possibility of the supernatural, the spirit world, an afterlife or a god. The universe certainly started 15 billion years ago in the Big Bang. The average empiricist does allow for the possiblity that things we have not yet discovered may exist. There might be powerful beings, aliens, universal membranes etc but we simply don't know yet.
Skeptical Atheist
The skeptic believes that by default divinity and the supernatural probably do not exist but he is willing to listen (very skeptically) to reason and evidence. The skeptic has probably ruled out all the world's religions as myth and hoax but is still willing to listen. The skeptic is also skeptical of scientific theory that is unprovable but less skeptical than he is of the supernatural. The universe may've started 15 billion years ago but we really can't say for sure or what the prime mover was. I suppose that I would belong in this description.
Spiritual Atheist
The spiritual atheist believes that there are absolutely, certainly unidentifiable events and movers but we cannot and maybe never will know what they are. The universe is a perfection of design but no known earth god was responsible - we simply do not know what powers did this. There may be or may not be an afterlife but prayer is pointless as the prime movers certainly didn't care to make themselves known so why would they want prayer? Deists, Buddhists and Unitarians qualify as Spiritual Atheists. (This may not sound like an atheist but this person will behave no different than any atheist.)
Local Atheist/Distant Agnostic
A localized atheist has come to the conclusion that his local religion is certainly wrong but hasn't made up his mind about the rest of the world.
True Agnostic
The True Agnostic has absolutely no opinion of the truth of any religion one way or another and probably doesn't care. Either this or he hasn't come to any concusions yet.
Local Agnostic
The Local Agnostic sees all of earth's religions except for his local religion as obvious mythology. He has his doubts about his own local relgion and either has no opinion or hasn't come to a conclusion yet or simply doesn't care.
New Age Theist - Believe ALL Religions
The New Ager has absolutely ZERO interest in truth, logic or reason. The New Ager just wants to smoke his spiritual bong while believing the idiocy that all religions have the same "basis" or "transcendence". Differences in religion are explained by human interference and only the best in every religion is brought together as some sort of spiritual kum-ba-ya. The fact that the world's religions are not just a LITTLE different but diametrically opposed in many, many ways doesn't really concern the spiritual stoner at all as facts just get in the way and harsh his mellow. A New Ager fully believes in Christianity, Buddhism and Nez Perce at the same time even though they openly contradict each other - this is no problem as contradictions only bother a rational mind.
Liberal Theist - Rewrite Your Religion
This bizarre offshoot will actually take the local religion, rewrite it, change all the basic tenets but still use its gods for some kind of authenticity. Basically a Liberal Christian will say "I believe in Jehovah but the Old Testament doesn't really say what it says and I don't believe in giants but I believe in angels but I don't believe in hell..." By changing the religion, the Liberal Theist has invalidated it but it saves him from answering the questions that STs ignore and FTs rationalize. Usually if the LT has the capacity to reason he will eventually take the next step of tossing out the whole religion.
Secularized Theist - Ignore Your Religion
The secularized theist is about 80% of the western world. Basically these are people who say they believe in their local religion but know almost nothing about it and ignore it for most of their lives. When reason conflicts with doctrine a Secularized Theist will usually choose reason and simply won't care that he's ignoring GOD'S WORD! When science or politics conflict with doctrine the ST will usually (but not always) side against doctrine. The ST is tolerant of all religions - if he can ignore his own then he can sure ignore Achmed's and sees Fundamentalist Theists as fanatics. It is entertaining that an ST usually sees atheists as immoral and will reflexively side with FTs against them.
Fundamentalist Theist - Believe Your Religion
These people believe every word of their local religion and will readily die for it. These are real god(s) with real power and their word must be accepted without question. When science, reason or politics conflict with doctrine, the FT will almost always side with doctrine in spite of personal hardship. FTs do not reason well and spend very little time thinking - instead they are only concerned with what their prophet, priest or minister has to say about a topic. FTs see STs as "Lukewarm" and Atheists/Agnostics as infidels that will be damned to hell.